1. Field of the Invention
This invention relates to multiprocessor systems and more particularly to a data link protocol for transmitting data packets between nodes in a multiprocessor system.
2. Background Information
Closely-coupled multiprocessor systems require high performance from the underlying communication network. It is not unusual to expect a constant level of performance from the communication network as the load generated by a plurality of attached processors within the system peaks. Traditionally, this demand has been met with backplane networks composed of buses of parallel wires controlled by high speed hardware units. While this interconnection methodology facilitates communication of parallel data at very high speeds, it is inherently unreliable because of the large number of wires and connectors involved in such a network. Moreover, the buses introduce crosstalk noise, signal skew and signal level shifting into the system which degrade its integrity and reliability. Additionally, the bus connectors introduce substantial reliability problems as a result of poor mechanical contacts between the bus and the connection target. These problems are increased by the repeated insertion and removal of connectors and movement of the system.
Alternately, the use of long haul networks have proven to be highly reliable. In long haul networks, a data link, such as a wire or optical fiber, which interconnects the communicating systems, facilitates the serial transmission of data packets from one system to another system. Long haul networks generally are characterized by fewer wires or fibers which actually connect the communicating systems, the feasibility of shielding wires to minimize crosstalk noise, reliable serial connectors and the use of sophisticated protocols which facilitate error detection and recovery on serial data links. Conceptually, backplane networks modeled after long haul networks would be an attractive solution to the reliability problem. Unfortunately, the model fails to provide the performance required by closely-coupled systems. Similarly, local area networks, which are capable of comparatively high speed data transmission over short distances, fail to provide the required performance. Local area networks and long haul networks perform the major portion of data transmission or network function with software which is resident in each of a plurality of communicating processors or host. This transmission methodology while adding flexibility to the system severely degrades the effective data transmission rate.